You can write a fixed 'baseurl', pointing to 'the old archive'
[fedora]
name=Fedora
baseurl=http://download.fedora.redhat.com/pub/archive/fedora/linux/releases/12/Everything/i386/os/
enabled=1
[updates]
name=Fedora-update
baseurl=http://download.fedora.redhat.com/pub/archive/fedora/linux/updates/12/i386/
enabled=1
... Or better, use a long term supported OS :
The EL 6 copy SL 6 is based on Fedora 12
http://www.scientificlinux.org/